366
M. Pal and M. Pradhan
losses, including waveguide losses due to light propagation in laser chip, mirror
losses originated from their configuration and losses from selection for a particular
emission wavenumber within the available gain curve.
The DFB-QCLs was first validated in 1997 by integrating a Bragg grating on
the top of the laser ridge along the light propagation direction [7]. The tunability of
the DFB-QCL can be achieved up to 5 cm
−1 by tuning the operational temperature
and/or injection current [8] and to overcome the limiting tunability of this type of
laser, multiple numbers of DFB-QCLs (DFB-QCL array) can be incorporated simultaneously [9–12]. Its narrowband single-mode emission makes it most preferable
laser source for gas spectroscopy and use of DFB-QCL array offers broad tunability
which is very much helpful for liquid-analytes analysis as well as for multiple trace
gas sensing. Next, incorporation of an external-cavity (EC) coupled with an external
diffraction grating in QCL chip overcomes the limited tunability of DFB-QCL and
offers several hundreds of wavenumbers of tuning range [13, 14].
This broad tuning range from the QCL chip is achieved by changing the angle
of the diffraction grating. The EC-QCLs are classified into three types depending
on their emission characteristics which are pulsed, standard continuous wave (CW)
and mode-hop-free (MHF)-CW configurations. But there is a disadvantage present
in using pulsed and continuous wave QCL in gas-phase spectroscopy due to modehopping phenomena during tuning [15]. Moreover, this unwanted phenomenon is
originated from competition between different optical modes to avail the net gain
in laser medium leading to distinctive gaps in the emission curve [16–18]. Thus, a
mode tracking system has been incorporated in the EC-QCL for exactly matching
the diffraction grating angle with external cavity length during the tuning process
[15]. However, the development and simultaneous evolution of QCL technology is
still a dynamic area of research.
3 QCL Spectroscopy for Trace Gas Sensing in Atmosphere
and Its Importance
The trace gases are defined as the gases that exist in small concentrations in a gas
mixture. Despite their low concentration, its quantitative detection has immense
importance in numerous fields such as in environmental sensing, trace amounts of
explosive detection and non-invasive medical diagnostics employing exhaled breath
analysis. Moreover, the invention and advancement of QCL play a paramount roll in
trace gas detection as it is capable of accessing entire mid-infrared molecular ‘fingerprint’ region (3–15 μm) and this spectral region is considered to be the choice for
high-sensitivity laser absorption spectroscopy [19, 20]. Since most of the important
trace molecules, being a simple or moderately complex structure, have fundamental
ro-vibrational transition frequencies lying in this spectral region (see Fig. 3). Thus,
the transition strengths are several orders of higher magnitude compared to the nearIR region leading to high sensitivity measurement [21]. The trace gases play an
M. Pal and M. Pradhan
losses, including waveguide losses due to light propagation in laser chip, mirror
losses originated from their configuration and losses from selection for a particular
emission wavenumber within the available gain curve.
The DFB-QCLs was first validated in 1997 by integrating a Bragg grating on
the top of the laser ridge along the light propagation direction [7]. The tunability of
the DFB-QCL can be achieved up to 5 cm
−1 by tuning the operational temperature
and/or injection current [8] and to overcome the limiting tunability of this type of
laser, multiple numbers of DFB-QCLs (DFB-QCL array) can be incorporated simultaneously [9–12]. Its narrowband single-mode emission makes it most preferable
laser source for gas spectroscopy and use of DFB-QCL array offers broad tunability
which is very much helpful for liquid-analytes analysis as well as for multiple trace
gas sensing. Next, incorporation of an external-cavity (EC) coupled with an external
diffraction grating in QCL chip overcomes the limited tunability of DFB-QCL and
offers several hundreds of wavenumbers of tuning range [13, 14].
This broad tuning range from the QCL chip is achieved by changing the angle
of the diffraction grating. The EC-QCLs are classified into three types depending
on their emission characteristics which are pulsed, standard continuous wave (CW)
and mode-hop-free (MHF)-CW configurations. But there is a disadvantage present
in using pulsed and continuous wave QCL in gas-phase spectroscopy due to modehopping phenomena during tuning [15]. Moreover, this unwanted phenomenon is
originated from competition between different optical modes to avail the net gain
in laser medium leading to distinctive gaps in the emission curve [16–18]. Thus, a
mode tracking system has been incorporated in the EC-QCL for exactly matching
the diffraction grating angle with external cavity length during the tuning process
[15]. However, the development and simultaneous evolution of QCL technology is
still a dynamic area of research.
3 QCL Spectroscopy for Trace Gas Sensing in Atmosphere
and Its Importance
The trace gases are defined as the gases that exist in small concentrations in a gas
mixture. Despite their low concentration, its quantitative detection has immense
importance in numerous fields such as in environmental sensing, trace amounts of
explosive detection and non-invasive medical diagnostics employing exhaled breath
analysis. Moreover, the invention and advancement of QCL play a paramount roll in
trace gas detection as it is capable of accessing entire mid-infrared molecular ‘fingerprint’ region (3–15 μm) and this spectral region is considered to be the choice for
high-sensitivity laser absorption spectroscopy [19, 20]. Since most of the important
trace molecules, being a simple or moderately complex structure, have fundamental
ro-vibrational transition frequencies lying in this spectral region (see Fig. 3). Thus,
the transition strengths are several orders of higher magnitude compared to the nearIR region leading to high sensitivity measurement [21]. The trace gases play an